The present invention relates to a method for processing conductive layer structures, and to devices including such a conductive layer structures such as reflective type liquid crystal display devices, and metal capacitors.
Such a method for processing, for instance used for processing a reflective type liquid crystal display device including four metal levels, is already known in the art, e.g. from U.S. Pat. No. 5,892,563 xe2x80x9cReflective Type Liquid Crystal Display Devicexe2x80x9d. Therein, in FIG. 1 a reflective type liquid crystal display device is shown including four r layers, of which the top metal layer serves as the pixel electrode layer, of which the lowest two layers serve as row and column electrodes, and or of which the third metal layer serves as a light shield for preventing incident light to disturb the operation of active circuitry within the substrate.
As is mentioned within this prior art document, the material used for the light shield is the traditional metal layer used in most CMOS processes, namely Aluminium.
Aluminium layers are traditionally deposited to thicknesses of typically 400 nm or more using standard CMOS processing steps. Since however the pixel electrode top layer has to be driven and consequently to make contact with the underlying second conductive layer, the presence of an intermediate thick metal-3 layer seriously increases the processing complexity for making such a contact.
The same is true in other conductive layer structures such as metal capacitors whereby the capacitance value of a metal2-metal3 capacitor is increased by parallel connecting it with a second metal-3-metal4 capacitor. In this case the metal4 top plate has to make contact with the metal2 bottom plate, whereas the intermediate metal3 plate serves as a common plate. The formation of such a deep contact between metal4 and metal2, in the presence of a thick metal-3 layer such as Aluminium, is difficult.
Straightforward using a thinner Aluminium layer does not present a solution to this problem since, as is also well known in the art, thinner Aluminium layers have problems with voids, such that their functioning as a light shield is hampered.
Another problem with Aluminium is that it is a light reflective layer. In reflective type liquid crystal display devices where the third conductive layer is used as a light shield, the incident light is thereby reflected multiple times in between the metal3 and metal4 layers. However this multiple reflected light can still penetrate the underlying silicon via some holes present in the light shield, e.g. for enabling the aforementioned contact between the fourth and the second conductive layers to be made.
An object of the present invention is therefore to provide a method for processing conductive layer structures of the above known type, as well as these conductive layer structures such as a reflective type liquid crystal display device and a metal capacitor, but wherein the aforementioned problems of topography, voiding and light reflection are solved.
According to the invention, this object is achieved by a method of processing conductive layer structures, including the steps of depositing a first conductive layer, a first intermetal insulator layer; a second conductive layer, a second intermetal insulator layer, a third conductive layer, a third intermetal insulator layer, and a fourth conductive layer, wherein the step of depositing the third conductive layer includes at least one succession of depositions of a light-absorbing layer on top of a light-reflective layer, such that the total thickness of said third conductive layer does not exceed 350 nm.
The object of the invention is further achieved by a reflective type liquid crystal display device (RTLCDD) including an array of reflective pixel electrodes over a surface of a silicon substrate with light transmissive regions located between the reflective pixel electrodes, structures defined in a first conductive layer between the reflective pixel electrodes and the surface, structures defined in a second conductive layer between the reflective pixel electrodes and the surface, and light shield structures defined in a third conductive layer between the reflective pixel electrodes and the surface, for preventing light entering at active circuitry located within the silicon substrate, the reflective pixel electrodes being formed in a fourth conductive layer, wherein the third conductive layer is composed of at least one multilayer comprising a light absorbing material on top of a light reflective material, such that the total thickness of said third conductive layer does not exceed 350 nanometers.
The object of the invention is still further achieved by a metal capacitor structure formed in a four-metal layer CMOS process wherein a first, a second, a third, and a fourth conductive layer are present, in between which a first, a second and a third intermetal insulator layer are present, the metal capacitor structure including a parallel connection of a first capacitor formed between the second conductive layer of the CMOS process and the third conductive layer of the CMOS process, and a second capacitor formed between the third conductive layer and the fourth conductive layer of the CMOS process, wherein the third conductive layer is composed of at least one multilayer comprising a light absorbing material on top of a light reflective material, such that the total thickness of the third conductive layer does not exceed 350 nanometers.
In this way, the presence of at least one multilayer consisting of a sandwich of a light absorbing material on top of a light reflective material, such that the total thickness of the third conductive layer does not exceed 350 nm, solves the aforementioned problems. Indeed, reducing the thickness with 12.5%, or more, results in less topography problems, whereas the presence of the absorbing layer on top of a reflective layer alleviates the problem of multiple reflections due to the extra absorption in the absorbing layer. Moreover, such a multilayer reduces the voiding.
Another characteristic feature of the present invention is that the light-absorbing layer comprises Titanium Nitride (TiN), on top of the light reflective layer which comprises Titanium (Ti).
Both materials are standard materials used in CMOS processing, and can be made very thin. As will become clear from the descriptive part of this document, a thickness of 60 nm of TiN on top of 20 nm of Ti can be used for this multilayer. In a particular embodiment of a reflective type liquid crystal display, this multilayer is repeated twice resulting in a total thickness of 160 nm for the third conductive layer which is sufficient for providing an effective shield for an envisaged application of shielding light with an energy of 1 Mlux. Depending on the envisaged applications which relate to the incident light in the structure, even thinner layers can be used. In the case of the metal capacitor structure, a single multilayer consisting of 60 nm TiN on top of 20 nm Ti can even be used. This presents a reduction in thickness of more than 80% with respect to the Aluminium layer.
Another characteristic feature of the invention is that the fourth conductive layer is composed of a light reflective layer on top of a light absorbing layer, and/or wherein the third intermetal insulator layer is further planarized to a thickness of maximum 365 nm above the third conductive layer, and/or where the fourth conductive layer has a thickness smaller than the dimensions of the light transmissive regions between the pixel electrodes, e.g., smaller than 500 nm, with the fourth conductive layer composed of a light reflective layer on top of a light absorbing layer.
This is especially important for the liquid crystal display devices whereby the problem of multiple reflections between the light shield and the pixel electrode layer is solved even better. Indeed, if the distance between the third and fourth conductive layers does not exceed the distance between the pixel electrodes, only incident light with wavelengths larger than the light transmissive regions between the pixel electrodes can enter the structure, and these will further be multiple reflected within a kind of tunnel structure between the third and fourth metal layers. Due to the presence of the light absorbing layer at both inner sides of this tunnel created by the bottom layer of the fourth conductive layer and the top layer of the third conductive layer, this light will be finally absorbed.
By keeping the openings between the pixel electrodes as well as the openings within the light shield smaller or equal to 500 nm, all incident light with larger wavelengths is prohibited from entering the structure. Thus only a small fraction of the light can enter the structure, it will be multreflected and finally absorbed between metal3 and 4. The thickness of the third intermetal insulator layer is thereby 365 nm at maximum, corresponding to the wavelength of UV light, which is the lower limit in wavelength of the entering light in some of the envisaged applications.
Yet another characteristic feature of the present invention is the second intermnetal layer between the second conductive layer and the third conductive layer has a thickness of a maximum of 600 nm.
Thereby, by keeping the dimension of the overlap region between the pixel electrodes and the underlying light shield to be at least a factor of 10 larger than the dimensions of the light transmissive regions between the pixel electrodes, enough of these multiple reflections will occur so that the incident lightwave is completely absorbed at the end of this tunnel between the light shield and the pixel electrodes.
Further characteristic features of the present are mentioned in claims 6-7, 14-15 and 18-19.
Thereby the thickness of the second intermetal insulator layer is also kept small enough, so that the total thickness of both second and third intermetal insulator layers allows one via hole to be formed and subsequently filled with a conductive material. A very small direct contact is thereby created between metal4 and metal2 level. This allows very dense structures to be made.